The present invention relates to a polyamide composition which comprises a specific amount of a polyamide mixture comprising two or more polyamides differing in terminal group concentration and a specific amount of a specific antioxidant. The invention further relates to a polyamide film for food packaging obtained from the composition. More particularly, the invention relates to a polyamide composition suitable for use in producing a film satisfactory in practical properties, e.g., the property of deteriorating little in mechanical properties or transparency upon retortion. This polyamide composition is suitable for use as a polyamide film for food packaging, especially a packaging film for retort pouch foods.
Polyamide films are excellent in various properties including gas barrier properties, toughness, freedom from pinholes, heat resistance, and oil resistance. Because of this, polyamides are used as packaging films, in particular, used as a base of single-layer films or laminate films or as a constituent material for multilayered films formed by coextrusion with other resins mainly in the field of food packaging.
In the step of packaging foods such as uncooked noodles, cooked foods, and salted vegetables, retortion has come to be frequently conducted in which the packages are treated for sterilization at a high temperature and a high humidity for the purpose of enabling the contents to be stored over long. Recently, higher temperatures of about 130xc2x0 C. tend to be used for the retortion. Films for food packaging are hence required not to change in mechanical properties or transparency upon treatment with hot water of steam having a temperature of about 130xc2x0 C.
There have been cases where polyamide films, when subjected to retortion under the conditions of a high temperature of about 130xc2x0 C. and a high humidity, suffer a considerable decrease in mechanical properties, in particular tensile strength, or suffer blushing to have impaired transparency.
Under these circumstances, investigations are being made in order to develop a polyamide which deteriorates little in mechanical properties, transparency, etc. upon high-temperature high-humidity retortion.
For example, a technique has been proposed in which a polyester film is laminated to a surface of a polyamide film so as to prevent the polyamide film from being directly exposed to a high-temperature high-humidity atmosphere. However, not only the properties of the polyamide film itself remain unimproved, but also this technique has drawbacks in productivity because the production steps necessitate a prolonged time period or are complicated.
JP-A-4-28727 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a packaging film for retort pouch foods which comprises a specific polyamide resin modified with terminal groups. However, even this polyamide film may be impaired in mechanical properties or transparency through retortion according to the conditions therefor and hence are usable in limited applications.
JP-A-7-268209 discloses a packaging film for retort pouch foods which comprises a polyamide composition containing a specific antioxidant. This polyamide film, however, is insufficient in mechanical properties and transparency after retortion.
Furthermore, JP-A-59-231089 discloses a polyamide composition containing a hydroxyphenylpropionate. However, no data are given therein on properties of this polyamide composition. In addition, neither a description concerning film uses nor a suggestion on influences of retortion on practical properties is given therein.
The present inventors made investigations in order to develop a polyamide for use as a material for a film which deteriorates little in mechanical properties, transparency, etc. upon retortion. As a result, it has been found that the object of the invention is accomplished with a polyamide composition comprising a polyamide mixture comprising two or more polyamides differing in terminal group concentration and a specific antioxidant. The invention has been completed based on this finding.
The invention provides, according to the first aspect thereof, a polyamide composition comprising
(A) 100 parts by weight of a polyamide mixture comprising two or more polyamides differing in terminal group concentration and
(B) from 0.01 to 1.0 part by weight of a hydroxyphenylpropionate represented by formula (1),
wherein the polyamide mixture (A) comprising two or more polyamides differing in terminal group concentration has a terminal amino group concentration of from 2.0xc3x9710xe2x88x925 to 7.0xc3x9710xe2x88x925 eq/g and a terminal carboxyl group concentration of from 1.5xc3x9710xe2x88x925 to 4.0xc3x9710xe2x88x925 eq/g, the terminal amino group concentration being higher than the terminal carboxyl group concentration, 
wherein R1 represents an alkyl group having 1 to 3 carbon atoms.
The invention further provides, according to the second aspect thereof, a polyamide film for food packaging obtained from the polyamide composition described above.